Diffraction by definition is the divergence of light from its initial line of travel.
But images cant diffract through walls, right? Diffraction depends on coherent sources of light waves from a monochromatic source. wrt the double slit ex. right?

Much more general than that. If the wavelength is anywhere close to the obstaces dimensions it will move around it, while small wavelengths are a bb on a brickwall, long wavelengths more like a a blob of jello which will peek around corners.